I was landing on 27R as this aircraft climbed away from 27l. It had a very clear trail of what looked rather like smoke from the left side of the aircraft. The FO commented that it looked as if they were on fire...
Perhaps 30 secoonds after it was airborne there were reports on the ground from other aircraft of a strong smell of fuel. It seems likely the gear would be up as without any indications the crew would have retracted the gear normally after getting airborne. As someone has said a fire would have led to the gear being dropped again.
Looked dramatic but by the sound of things was not. Happily...
Regards
D